Any single-core processor, even a relatively high-clocked one(*) is going to feel slow for quite a few things in normal day to day uses on Windows, and even for some things on Linux. Blame software bloat, routine use of multithreading (including in IE8), and much more complicated web apps.

Of course, if someone were sticking with XP, IE 6/7, and Office 2003, and never used the present generration of very javascript-heavy web sites (ie Facebook), it's probably fine...

(* and a 1.6ghz AMD isn't high clocked, although it's going to be faster than a 1.6ghz Atom, but slower than a 1.6ghz Core Solo and probably slower than a 1.6ghz present-generation Celeron. Put differently, it's about the same speed as the 2.4ghz Pentium 4 I had on my desktop back in 2003.)